In electron beam welding, an electron gun produces from a heated cathode a stream of electrons which is accelerated and focused onto a workpiece. Electron beam welding is extremely advantageous where deep penetration and a narrow heat-affected zone are required but a suitable electron beam generator requires a very high accelerating voltage, typically 150 kV is used. In U.S. Pat. No. 4,020,318 describe an electron beam generating system in which the auxiliary supply for as least one electrode of the electron gun (for example the filament or biassing electrode) is provided by a high frequency oscillator connected to a tansformer or other coupling means, the output of which is applied as the electron beam gun column to a component which is itself connected to the received output of the high voltage generator. The advantage of this arrangement is that it dispenses with the superimposition of the auxiliary supply on the HT (High Tension) voltage at a remote high voltage generator and with the corresponding auxiliary supply conductor in the conventional high voltage cable between the high voltage generator and the electron beam gun column. This system has worked well for filament and bias supplies for a simple triode gun but the control of the system for an indirectly heated or "back-bombardment" type of gun is complex.
In a back-bombardment system, the filament generates a stream of electrons which are accelerated onto a target (cathode) which is thereby heated and energised to generate its own stream of electrons which is then accelerated though an anode onto the workpiece. In order to control the system, it is first necessary to set up the back-bombardment system. This involves causing current to flow through the filament and then to monitor the electron flow to the target (which is held at a positive voltage of the order of 1 kV relative to the filament) until the correct operating filament emission current is reached. The procedure is known as "peaking" the filament. Thereafter, the main beam current from the target cathode to the workpiece must be optimized by adjusting the back bombardment accelerating voltage onto the target. Not only must this process be carried out at start-up, but also during use since the filament itself will gradually thin necessitating adjustment of the filament current to avoid filament overheating and failure of the primary filament.